It is often desirable to separate by distillative techniques a gas feed stream, such as one derived from natural gas or from petroleum or refinery operations, into one or more component fractions, either to recover a particular fraction or to remove an undesirable fraction from the gas stream. Typically, such distillation techniques involve introducing a gaseous, liquid or mixed-phase feed stream into one or more distillative columns containing vapor-liquid-contacting devices, such as trays, packing devices or combinations thereof, with the distillation temperature and pressure remaining at desirable conditions. An overhead product stream is removed, containing the lighter fractions, and a portion of the stream is at least partially condensed in a condenser and is recycled as a reflux stream, while a bottom product stream is removed, containing heavier components, and at least a portion of the stream, typically heated in a reboiler and recycled for use in the bottom of the column. The overhead product stream and the bottom product stream may be recovered and used or, if desired, sent for further separating or processing.
It has been found desirable also to introduce an additive agent particularly into the upper portion of the distillation column, in order to alter the distillation conditions, so as to enhance the recovery of the overhead or bottom products. For example, in U.S. patent application Ser. No. 94,226, filed Nov. 14, 1979 (now U.S. Pat. No. 4,318,723, issued Mar. 9, 1982), it was discovered that the use of a liquid additive agent in the cryogenic distillation separation of methane from a gas feed stream containing acid gas components was particularly effective for separating methane from high carbon-dioxide-content feed in a single distillative column, without the formation of carbon-dioxide solids. A liquid additive agent may comprise, for example, C.sub.3 -C.sub.6 alkanes or mixtures thereof, sulfur dioxide or sulfur trioxide, and particularly butane-plus, as effective liquid additive agents to be introduced into the distillative column. The introduction of the liquid additive agent in the upper portion of the column and into the upper portion of the solids potential zone provided for the recovery of an overhead product stream of methane, and the removal of a bottom product stream containing carbon-dioxide methane and higher hydrocarbons and, optionally, hydrogen sulfide.
It also has been found that the use of additive agents may be employed usefully in the separation of acid gases, such as carbon dioxide, from ethane or from higher hydrocarbons in distillation processes, where normally the carbon dioxide and the ethane would form an azeotrope which would make distillation recovery of high fractions of the carbon dioxide difficult. For example, U.S. Pat. No. 4,350,511, issued Sept. 21, 1982, is directed to a method of separating carbon dioxide from a gas-stream mixture containing carbon dioxide and ethane, and wherein the distillative column is operated under conditions of pressure, temperature and composition to produce an enriched carbon-dioxide overhead product stream and an enriched ethane bottom stream. In the method, a liquid additive agent is introduced into the upper portion of the distillative column and in an amount sufficient to provide for a relative volatility of greater than 1 of the carbon dioxide to the ethane at or below the point of introduction of the liquid additive agent, thereby altering the possible formation of the azeotrope and permitting a carbon-dioxide-enriched overhead stream to be recovered.
Further, another use of a liquid additive agent, in order to increase the relative volatility of acid gas components, such as carbon dioxide and hydrogen sulfide, in a distillation technique is described in U.S. Pat. No. 4,293,322, issued Oct. 6, 1981. A distillative separation of carbon dioxide and hydrogen sulfide is shown to be improved by adding a liquid additive agent to the distillative column, which increases the relative volatility of the carbon dioxide to hydrogen sulfide, thereby increasing the separation efficiency, so that a carbon-dioxide fraction is removed in the overhead stream and a hydrogen-sulfide-enriched fraction and the liquid additive are removed with the bottom product stream.
In all of the foregoing patents, all hereby incorporated by reference, an additive agent is employed, particularly a liquid additive agent, and preferentially derived from one of the components in the feed stream, and is used to enhance separation efficiency. The liquid agent typically is miscible within the column and is at a point in the column below the column top. Also in all of the illustrations, the additive agent is usually removed, and the bottom product, or a portion thereof, is recycled back for use in the column or in other columns. If is, of course, desirable to improve such distillation techniques where additives are employed, to save energy and to increase the efficiency of the desired separation.